Lake Tahoe is “the fairest picture the whole earth affords,” Mark Twain once wrote. Its crystal blue waters, surrounded by stunning snowy mountains, define one of California’s crown jewels as an American landmark. It attracts 3 million skiers, boaters, campers, hikers and other visitors each year.

But it could look very different in 100 years.

Climate change could profoundly affect the Tahoe area, scientists say, taking the snow out of the mountains and the blue out of the water.

Last winter’s ski season showed a glimpse of what a future, warmer Tahoe may look like. Snow didn’t start falling in the mountains until January. The California Ski Industry Association reported that 25 percent fewer skiers visited the Sierra last season. For a region that boasts a $5 billion year-round economy, that hurts.

New climate models show that in a worst-case scenario average temperatures in the Tahoe area could rise as much as 9 degrees Fahrenheit by the end of the century. That’s equivalent to moving Lake Tahoe from its current elevation of 6,200 feet above sea level to 3,700 feet, climate scientists report in a special January issue of the journal Climatic Change. That’s as high as the peak of Contra Costa County’s Mount Diablo, which gets only an inch of snow a year.

Simulations also suggest that the percent of precipitation that falls as snow may decrease to as little as 10 percent by the end of the century. It’s now 30 percent. In time, the snow line will move up the mountains, said Robert Coats of UC Davis, a co-editor of the special issue.

Resorts higher up in the mountains, such as Heavenly, Squaw Valley and Alpine Meadows, will likely keep their snow, he said. “In fact, they may actually benefit in the short run as the snow disappears from lower elevations.”

Lower resorts face issues

Homewood Ski Resort, a lower-elevation resort without an extensive snowmaking system, is well aware of the threat of climate change. Last season, Homewood didn’t open until Dec. 14, said resort spokesman Paul Raymore, and it wasn’t able to open any chair lifts until January. More winters such as last year’s would be disastrous. “We do rely on Mother Nature and what she provides in terms of natural snowfall,” Raymore said.

While doing little to curb global climate change, the resort does encourage skiers to use public transit, now offering $5 off lift tickets for those who do. “We have a vested interest in ensuring that the mountains stay cold,” Raymore said.

To be sure, people should keep in mind that the climate models aren’t necessarily forecasts, said Michael Dettinger, a climate modeler at Scripps Institute of Oceanography in San Diego and one of the authors of the special Climatic Change issue. “They’re what-if predictions,” he said, adding that scientists can’t say yet which scenario is most likely to unfold.

But even under the most optimistic conditions, Dettinger said, climate change will become a problem for the Tahoe region to deal with on the same scale as other pressing environmental issues, such as the invasive Asian clam, which scientists have been battling for a decade.

It’s not just the mountains that would look different in a warmer climate, according to Climatic Change. The worst-case scenarios also predict a devastating ecological collapse of the lake and loss of its signature clarity and blue color.

Keeping water mixed

Many lakes undergo a process every year, or every few years, that keeps the lake water well-mixed. As water temperature changes through the seasons, it creates circulation in the lake. The warm water on top of the lake in summer cools off in the fall and sinks, mixing with cold deep water. In a warmer climate, the surface water won’t cool off enough to mix with deeper water.

If mixing doesn’t happen, then oxygen from the surface doesn’t reach the lake floor. Without oxygen, the chemistry of the lake floor changes, and nutrients like phosphorus and nitrogen are released into the water. Excess nutrients lead to algal blooms which, as any swimming pool owner knows, turn the water from clear blue to a murky green.

It’s a serious threat, Climatic Change authors say, “to the characteristics of the lake that are most highly valued.”

“The lake more or less crashes,” Dettinger said. “It goes to a bad place and doesn’t return.”

The Tahoe Regional Planning Agency’s job is to respond to today’s challenges and anticipate tomorrow’s in partnership with the U.S. Forest Service and city, county and state governments. The agency, created by California and Nevada in 1969, regulates every development in the Tahoe area to ensure sustainable growth.

Threats to the lake’s ecosystem — including climate change — pushed the agency to update its regional plan, which had been in place since 1987, said agency spokesman Jeff Cowen. The new 2012 regional plan focuses on restoring lake water quality and redeveloping “green” town centers, Cowen said. It also calls for updates every four years and includes a “sustainable communities strategy” to meet California’s standards for greenhouse gas emission reduction in the region.

Some years wet, some dry

Frank Gehrke, a snow surveyor for California’s Department of Water Resources, has been working in the Sierra for more than 30 years. He’s seen his share of wet years and dry years. “The overall trend is overwhelmed by the difference from year to year,” he said.

Although last winter’s ski season was hard on the resorts, the previous season boasted early heavy snow — and skiing lasted until the Fourth of July.

Now that the best- and worst-case scenarios have been modeled, scientists will now refine their models further to pin down the most likely climate scenario. “I think it’s going to be pretty hard to put a date on when there will be no snowfall at all in the Tahoe Basin,” Coats said. “I think that may be pretty far out in the future.”

This season is off to a great start, the state’s first Sierra Nevada water survey revealed this past week. Homewood opened before Halloween, thanks to early snowstorms. But good years come and go. And Mark Twain’s vision of Lake Tahoe may not last forever.